An energy efficient oxygen concentrator for filling high pressure portable cylinders with medical oxygen for use by ambulatory patients. Two compressors provide two pressurized air sources, one for operating an oxygen concentrator to provide a stream of oxygen enriched gas, and the other for driving a pressure intensifier for filling portable oxygen cylinders. Pressurized exhaust from the pressure intensifier is returned to the inlet side of at least one of the compressors for reducing the energy required to drive the compressor. Preferably, each compressor has a single reciprocating piston, a single motor drives both pistons and the pressurized exhaust from pressure intensifier is provided to the inlets for both compressors.